Chemically reducing residual styrene monomer in styrene polymers and shaped products formed therefrom

ABSTRACT

A process for regulating free styrene monomer in styrenic polymers during melting which involves having an effective amount of a styrene scavenger such as myrcene in admixture with the polymer during melting. Shaped products such as pellets, preforms, containers, film, sheet, etc., formed from such melt are well suited for use in packaging food, beverages, pharmaceuticals, cosmetics, etc., in that residual styrene monomer in the walls of such products is reduced.

CROSS REFERENCE TO RELATED APPLICATIONS

1. "Chemically Reducing Residual Acrylonitrile Monomer In NitrilePolymers and Shaped Packaging Materials Formed Thereby," E. F. Tokas,U.S. Ser. No. 943,565, filed Sept. 18, 1978.

2. "Molding Compositions and Process for Preparing Same," E. F. Tokas,U.S. Ser. No. 951,493, filed Oct. 16, 1978, now U.S. Pat. No. 4,180,486.

BACKGROUND OF THE INVENTION

This invention relates to a process for chemically reducing styrene (S)monomer in styrenic polymers and more particularly to products such aspckaging materials shaped from the melted polymer wherein free S monomertherein is reduced.

Thermoplastic polymer compositions containing polymerized S can beshaped into a wide variety of useful articles by conventional techniquessuch as extrusion, milling, molding, drawing, blowing, etc. Applicationsfor such shaped articles are widespread and include structural unitswhere properties such as low thermal deformation, impact resistance(when a rubber component is present) and high gloss are required. Whenacrylonitrile is present with styrene in a copolymer at a concentrationof at least about 50 weight percent, the compositions uniquely exhibitexcellent solvent resistance and low permeability to liquids and gaseswhich make them especially useful as a lightweight substitute for glassin packaging and particularly in the manufacture of bottles, film,sheet, tubs, cups, trays and other containers for liquids and solids.

In manufacturing such polymer compositions, it is well known that free,unconverted, S monomer remains absorbed within the polymer particleswhen polymerization is not 100 percent complete and which is thereforepresent in products formed therefrom. Recently government regulatoryagencies are moving toward establishing maximum permissible levels ofvarious monomers in the environment on grounds that excess levels mayconstitute a health hazard, and particularly regulations have beenapplied to packaging materials intended for contact with environmentallysensitive products such as food, beverages, pharmaceuticals, cosmeticsand the like for which application styrenic copolymers are especiallysuited.

Even though possible to reduce residual styrene monomer (RST) instyrenic polymers before melt processing via stripping, it has beendiscovered that free S monomer is thermally regenerated due to polymerunzipping during melting at standard melt processing times andtemperatures. Depending on the level of S present in the polymer beforemelting, such an increase could be sufficient to (i) be extractable fromshaped packaging materials in contact with food simulating solventsand/or (ii) be released into the atmosphere around melt processingequipment. Accordingly, reduction of S monomer in the raw polymer couldbe ineffectual in controlling S in shaped products formed in anoperation involving melt processing.

SUMMARY OF THE INVENTION

Now, however, process improvements have been developed to minimize suchprior art shortcomings.

Accordingly, a principal object of this invention is to provide processimprovements to minimize generation of free S monomer during conversionto melt form of a polymer composition containing polymerized styrene.

Another object is to provide a chemical scavenger for or regulator of Smonomer which is effective at relatively low levels of S monomer tominimize generation of the latter during melt processing ofpolymerized-styrene-containing polymers.

A further object is to provide such a scavenger which is compatible withthe polymer at low concentrations to avoid the need to use excessamounts which could cause deterioration of properties in the shapedproduct such as optical (e.g. color and haze) and taste properties whichare important in packaging environmentally sensitive materials.

A specific object is to provide a scavenger which not only regulates thelevel of RST in the styrenic polymer but also regulates the level offree acrylonitrile monomer when polymerized acrylonitrile is present asa constituent of the styrenic polymer.

Other objects of this invention will in part be obvious and will in partappear from the following description and claims.

These and other objects are accomplished in the process of melting astyrenic polymer by physical working while generating free styrenemonomer by providing the improvement which comprises having a scavengingamount of myrcene in admixture with the polymer during such melting tominimize the level of free S monomer in the polymer.

From a product standpoint, a shaped article is provided which is formedof a thermoplastic polymer comprising at least about 10 weight percentof polymerized styrene monomer wherein walls thereof contain thereaction product of myrcene and styrene monomer.

PREFERRED EMBODIMENTS

Styrenic polymers useful in the present invention generate free Smonomer during melting and may constitute either a styrene homopolymeror a copolymer containing at least about 10 percent by weight ofpolymerized S together with one or more copolymerized comonomers. Suchmonomers include:

(a) monovinylidene aromatic hydrocarbon monomers other than styrene ofthe formula: ##STR1## wherein R¹ is hydrogen, chlorine or methyl and R²is an aryl group of 6 to 10 carbon atoms and may also containsubstituents such as halogen as well as alkyl groups attached to thearomatic nucleus, e.g., alpha methylstyrene, vinyl toluene, alphachlorostyrene, ortho chlorostyrene, meta chlorostyrene, parachlorostyrene, ortho methylstyrene, para methylstyrene, ethyl styrene,isopropyl styrene, dichloro styrene, vinyl naphthalene, etc. (b) loweralpha olefins of from 2 to 8 carbon atoms, e.g., ethylene, propylene,isobutylene, butene-1, pentene-1 and their halogen and aliphaticsubstituted derivatives, e.g., vinyl chloride, vinylidene chloride, etc.

(c) acrylic acid and methacrylic acid and the corresponding acrylate andmethacrylate alkyl esters where the alkyl group contains from 1 to 4carbon atoms, e.g. methyl acrylate, ethyl acrylate, propyl acrylate,methyl methacrylate, etc.

(d) vinyl esters of the formula: ##STR2## wherein R³ is hydrogen, analkyl group of from 1 to 10 carbon atoms, an aryl group of 6 to 10carbon atoms, e.g., vinyl formate, vinyl acetate, vinyl propionate,vinyl benzoate, etc.

(e) vinyl ether monomers of the formula:

    H.sub.2 C═CH--O--R.sup.4

wherein R⁴ is an alkyl group of from 1 to 8 carbon atoms, an aryl groupof from 6 to 10 carbon atoms or a monovalent aliphatic radical of from 2to 10 carbon atoms, which aliphatic radical may be hydrocarbons oroxygen--containing, i.e., an aliphatic radical with ether linkages andmay also contain other substituents such as halogen, carbonyl, etc.Examples of these monomeric vinyl ethers include vinyl methyl ether,vinyl ethyl ether, vinyl n-butyl ether, vinyl 2-chloroethyl ether, vinylphenyl ether, vinyl cyclohexyl ether, 4-butyl cyclohexyl ether, andvinyl p-chlorophenylene glycol ether, etc.

(f) olefinically unsaturated mononitriles having the formula: ##STR3##wherein R⁵ is hydrogen, an alkyl group having 1 to 4 carbon atoms or ahalogen. Such compounds include acrylonitrile, methacrylonitrile;ethacrylonitrile; propioacrylonitrile, alpha chloroacrylonitrile, etc.

Additional comonomers useful in the practice of this invention are thosecontaining a mono- or di-nitrile function. Examples of these includemethylene glutaronitrile, 2,4-dicyanobutene-1, vinylidene cyanide,crotonitrile, fumaronitrile, maleonitrile.

Preferred comonomers are the olefinically unsaturated mononitriles,monovinylidene aromatic hydrocarbons, lower alpha olefins, acrylic andmethacrylic acid and the corresponding acrylate and methacrylate esters,with the olefinically unsaturated mononitrile hydrocarbons being moreparticularly preferred. Most specifically preferred is acrylonitrile andalpha methylstyrene.

The amount of comonomer as defined above present in the styreniccomposition can vary up to about 90% by weight based on the total weightof the styrenic polymer composition. Preferred styrenic compositions forpackaging applications requiring excellent oxygen and water vaporbarrier properties in the packaging materials contain from about 10 toabout 50% by weight of polymerized styrene monomer and from about 90 toabout 50% by weight of polymerized acrylonitrile co-monomer and morepreferably from about 15 to about 45% by weight of styrene monomer andfrom about 85 to about 55% by weight of acrylonitrile monomer, all basedon total polymer weight.

Styrenic polymers within the scope of this invention may also contain anelastomer in the form of a synthetic or natural rubber component such aspolybutadiene, isoprene, neoprene, nitrile rubbers, natural rubbers,acrylonitrilebutadiene copolymers, ethylene-propylene copolymers,chlorinated rubbers, etc., which are used to strengthen or toughenproducts such as shaped packaging materials. This rubber component maybe incorporated into the styrene polymer by any of the methods wellknown to those skilled in the art, e.g., direct polymerization ofmonomers, polyblends, grafting the styrene-acrylonitrile monomer mixtureonto the rubbery backbone, etc. Especially preferred are polyblendsderived by mixing a graft copolymer of acrylonitrile and comonomer onthe rubbery backbone with another copolymer of acrylonitrile and thesame comonomer. Generally, such rubber component may comprise from 0 toabout 25% and preferably up to about 10% by weight of the nitrilepolymer composition.

The active treating ingredient for reducing the free S monomer contentof the styrenic polymer is myrcene having the formula 3methylene-7-methyl-1,6 octadiene. Myrcene is a terpene hydrocarbon whichhas U.S. Food and Drug Administration (FDA) approval as a food additive.It has the form of a colorless liquid at room temperature having aboiling point of 67° C. and a balsomic resinous odor. In place of purenatural myrcene which occurs as a constituent of bay oil and otheressential oils, or pure synthetic myrcene, it is possible to employmaterials rich in myrcene. For example, the pyrolysis of beta pineneyields mixtures containing as much as 77 weight % myrcene in conjunctionwith minor quantities of limonene and other complex terpenes.

The amount of myrcene used in the present invention is sufficient tominimize free residual styrene monomer (RST) in the styrenic polymerafter melting, and in instances where the polymer end use is forpackaging environmentally sensitive substances such as food,pharmaceuticals, margarine, butter and the like, should not besufficient to impart a myrcene-related taste to any such packagedsubstances. Allowance should be made for vaporization of some of theadmixed myrcene during melting of the styrenic polymer. In general theregulating amount of myrcene required to minimize RST will be in therange of from about 0.005 to about 2.0% by weight based on the weight ofthe styrene polymer. In most applications the amount of myrcene will bein the range of from about 0.01 to about 1.5% by weight. When using acompound which yields myrcene or else contains or behaves as a myrcenedonor, the amount of such myrcene compound is calculated so as toprovide an amount of myrcene within the preceding ranges.

As stated, styrenic polymers having reduced free, residual styrenemomomer therein after melting are prepared by a process which compriseshaving a scavenging amount of myrcene in intimate admixture with thestyrenic polymer during its conversion from solid to melt form. Theintimate mixture of myrcene and styrene polymer may be obtained bycombining the myrcene with the styrene polymer by adding the myrcenecompound to the polymerization system during preparation of the styrenicpolymer. Alternatively, the myrcene compound can be combined with thestyrenic polymer after polymerization, as for example, by adding it tothe styrenic polymer in the polymer recovery steps such as duringcoagulation, stripping, washing, drying, etc., or by steeping thepolymer in the presence of the liquid myrcene compound. Another methodis to dry blend the styrenic polymer and myrcene prior to a melting stepused in compounding or shaping the polymer. In still another method themyrcene and styrenic polymer are dispersed in a liquid medium followedby evaporation of the liquid medium. In still another method, themyrcene is injected into the molten polymer during the meltingoperation. Other methods of forming the intimate mixture will becomeapparent to those skilled in the art upon reading the presentspecification.

In general, the preferred method of admixing the styrenic polymer andmyrcene is blending the styrenic polymer in dry form with the myrcene.

The forming operations used to prepare products within the scope of thisinvention such as the preferred polymeric packaging materials, e.g.,sheet, tubs, trays, containers such as bottles, cans, jars, etc.,preforms for forming same and the like, are carried out by shaping thestyrenic polymer by procedures known in the art. In this connection,conventional plasticators can be used utilizing a screw rotating withina plasticizing zone to masticate the polymer wherein the screw is eitheraxially fixed or reciprocable, the latter occurring for example in aninjection molding system. The forming operations include both a meltingstep wherein the styrene polymer is converted from solid to melt form byphysical working at temperatures in the range of from about 177° toabout 262° C. and any subsequent shaping step performed on the melt. Themelting and shaping steps may occur sequentially or substantiallysimultaneously. Examples of forming operations used to prepare shapedpolymeric packaging materials include pelletizing, extrusion, blowmolding, injection molding, compression molding, mill rolling, vacuumforming, plug assist thermoforming from sheet material, combinations ofthe foregoing and the like.

More than one forming operation wherein the styrenic polymer is heatedand shaped may be involved in certain instances. This can occur, forexample, in the case of a polymer which is heated through working to amelted state, extruded and pelletized and then the pellets are heatedagain for melting and shaping into sheet or a tubular parison which isthen shaped into a container such as a shallow tub or a bottle. In suchsituations, the present invention contemplates having the myrcene incontact with the styrenic resin during at least one of the formingoperations wherein the polymer is heated to the point where it melts andthen is shaped. Preferably when more than one heating step is involved,the myrcene compound is intimately admixed with the styrenic polymerbefore or during the first step wherein the polymer is heated to thepoint where it melts.

The actual measurement of the amount of RST in products formed ofstyrenic polymers is not considered part of this invention. In thisregard any method capable of detecting S monomer (and acrylonitrilemonomer where pertinent) to about 1 ppm can be used. Such methods, whichare known to those in the art, include colorometric, polarographic, gaschromatographic, fluorometric and electrochemical measurements with gaschromatographic being preferred. Methods for measuring RAN are publiclyavailable from the U.S. Food and Drug Administration and are describedin and a part of Food and Drug Administration Regulation No. 121.2629which is referenced in Vol. 40, No. 30 of The Federal Register.

The present invention also contemplates the use of other additives andingredients in the polymeric compositions which do not adversely affectthe properties of the resulting molded products such as taste when suchproducts are packaging materials intended for food contact use. Examplesof these ingredients include thermal stabilizers, light stabilizers,dyes, pigments, plasticizers, fillers, antioxidants, lubricants,extrusion aids, scavenger(s) for monomers other than styrene, etc.

The following examples are set forth in illustration of the inventionand should not be construed as limitations thereof. All parts andpercentages are by weight unless otherwise specified. Wheneverconvenient, residual styrene is abbreviated as RST. The amount ofmyrcene compound is based on the weight of the styrenic polymer. A dashin a Table means that particular product was not tested.

EXAMPLES 1 AND 2

A styrenic copolymer in bead form containing 32% polymerized styrene and68% polymerized acrylonitrile prepared by conventional aqueouspolymerization methods was dry blended with various scavenging amountsof myrcene. Blending to insure intimate admixture of the polymer andmyrcene was carried out by placing the quantities of each in apolyethelene bag and shaking vigorously for two minutes. The styrenicpolymer blends were converted from solid to melt form by physicallyworking the polymer in multiple pass extrusion with a conventionalplasticator in the form of a one inch extruder having a 16:1 L/D ratiofitted with a rotary two-stage screw operating at 70 RPM. Extrudatestrands issuing from the die were cut into pellet form. Zonetemperatures of the extruder were set to provide a melt stocktemperature of approximately 218° C. After each pass, samples wereanalyzed for RST content with the results of these tests tabulated inTable I below.

                  TABLE I                                                         ______________________________________                                        SUMMARY OF EXAMPLES 1 AND 2                                                            ppm RST                                                                    Percent              One   Two    Three                                 Ex.   Myrcene  Polymer Beads                                                                             Pass  Passes Passes                                ______________________________________                                        1     0.0      7.0         24.0  27.5   30.0                                  2     0.97     7.0         16.0  --     21.7                                  ______________________________________                                    

Control Example 1 illustrates that without myrcene the level of RST inthe polymer during melt processing increases about these to four folddepending on the number of extrusion passes, vis-a-vis the initial RANlevel in the polymer. Example 2 illustrates that the use of a smallscavenging amount of myrcene effectively suppresses and regulates theamount of RST in the polymer at significantly lower levels than thecontrol at very dilute concentrations of the additive and the ST in thepolymer.

EXAMPLES 3 TO 5

The following examples 3 to 5 illustrate the present invention inproviding shaped, e.g. molded, products with regulated RST content whichare intended for packaging environmentally sensitive substances.

In these examples 3 to 5, the styrenic polymer initially in pellet formis one which is modified for impact resistance (high impact polystyrene)and comprises a polyblend of polystyrene and about 8 percent (based onpolystyrene weight) of a dispersed rubber phase comprising a rubbergrafted with polymerized styrene wherein the rubber is dispersed asparticles having a size between about 1 to 20 microns and which containabout 1 to 5 parts of grafted and occluded polystyrene per part ofrubber. Such pellets were combined with liquid myrcene and blended in atumbler for one hour. In Example 4, the admixture was converted to meltform and injection molded directly into flat rectangular chips 3 inches(7.6 cm.)×4 inches (10 cms.)×100 mils thick on a laboratory size Arburgrotating-reciprocating screw injection molding machine operating at800-1000 psi back pressure with a stock temperature of about 232° C. InExample 5, the admixture of polymer and myrcene was first melt blendedvia a rotating screw-plasticator and shaped into pellets by cutting thestrand extrudate, with such pellets then being remelted and injectionmolded into such chips. Five gram samples from the chips were dissolvedin dimethyl formamide and tested for RST. The results of these tests aretabulated in Table II below.

                  TABLE II                                                        ______________________________________                                        SUMMARY OF EXAMPLES 3 TO 5                                                              ppm RST                                                                             Molded RST Increase                                           Ex.   % Myrcene Pellet    Chip   (ppm)   %                                    ______________________________________                                        3     0.0       826       1001   +175    21.2                                 4     0.5       836       955    +119    14.2                                 5     0.5       845.sup.(1)                                                                             897    + 52     6.2                                 ______________________________________                                         .sup.(1) After first melt blending                                       

The above data illustrates that myrcene reduces the level of RST inshaped products formed from styrenic resins according to the inventionover that present (Ex. 1) when no myrcene at all is used. The RST levelin the pellets of Example 5 is higher than that of Example 4 because ofthe heat history associated with the melting step in obtaining thepellets of Example 5 which was not present in Example 4. The RST levelin the molded chips of Example 5 is lower than that of Example 4 becauseof the better dispersion of the myrcene in the polymer achieved inremelting the previously melt blended pellets during molding of thechips of Example 5.

To the best of applicant's knowledge, fabrication of other forms ofshaped materials such as those used in packaging environmentallysensitive substances, for example, film or sheet material useful asoverwrappings, tubs or cuplike containers for holding margarine, butterand similar based food products, as well as trays for meats, poultry,etc., should give similar results to those of Examples 4 and 5 in termsof minimizing RST buildup for comparable levels of polymerized styrenein the starting resin.

EXAMPLES 6 TO 9

The following examples 6 to 9 illustrate a feature of the invention inthe use of myrcene in controlling free, residual acrylonitrile monomer(RAN) in styrenic polymers containing polymerized acrylonitrile as wellas polymerized styrene. The polymer composition is identical to those ofExample 1, 2 with extrusion conditions being somewhat higher at about254° C. The results of these tests are tabulated in Table III below.

                  TABLE III                                                       ______________________________________                                        SUMMARY OF EXAMPLES 6 TO 9                                                    Percent    ppm RAN                                                            Ex.  Myrcene   Polymer Beads                                                                             1 Pass                                                                              2 Passes                                                                             3 Passes                              ______________________________________                                        6    0.0       0.7         7.0   9.9    11.7                                  7    0.97      0.7         2.5   2.6    3.0                                   8    0.14      0.7         6.0   8.0    7.1                                   9    0.03      0.7         7.3   --     9.9                                   ______________________________________                                    

Control Example 6 illustrates that without myrcene the level of RAN inthe polymer during melt processing increases about ten to sixteen folddepending on the number of extrusion passes, vis-a-vis the initial RANlevel in the polymer. Examples 7-9 illustrate that the use of smallscavenging amounts of myrcene, (e.g., between about 0.005 to 2.0percent) effectively suppresses and regulates the amount of RAN in thepolymer at significantly lower levels than the control at very diluteconcentrations of myrcene and AN monomer in the polymer.

As an aid in reducing problems associated with feeding thepolymer-myrcene mixture to the rotating screw plasticator, about 1 to 15percent (based on the weight of the styrene polymer) of an antislipagent in the form of a coagulated and dried rubber crumb latex may bemixed with the polymer-myrcene mixture before charging the extruder. Arubber crumb successfully so used comprises by weight: butadiene 30-34%;styrene 38-42%; acrylonitrile 16-20%; methyl methacrylate 8-10% andethylene glycol dimethyl stearate 0.5-1%.

EXAMPLES 10 TO 14

As an extension of examples 6 to 9 the following examples 10-14illustrate the invention in providing products shaped from styrenicpolymers containing polymerized acrylonitrile with low RAN content whichare intended for packaging environmentally sensitive substances. Styrenepolymer of the type used in examples 1, 2 is dry blended with 1 percentmyrcene compound mixed with the rubber crumb mentioned in connectionwith examples 6 through 9, and then injection molded in a commercialsize injection molding machine at temperatures in the range of from 230°to 270° C. into hollow, tubular preforms of circular cross section,closed at one end and having a molded finish at the other end. Suchpreforms are further described in U.S. Pat. No. 3,900,120, col. 5, lines13-42, the content of which is incorporated herein by reference. Thepolymer of the walls of such preforms and bottles is considered tocontain the reaction product of (a.) an AN and ST chemical scavenger,i.e., myrcene and (b.) both AN and ST monomer. After cooling to roomtemperature, preforms formed as just described were analyzed for RANcontent while others formed in such manner were reheated to molecularorientation blowing temperature of about 132° to 138° C. and thendistended in a conventional blow mold into 32 ounce (950 cc.)self-supporting bottles intended for packaging beverages and the like.The level of RAN in the bottles was then determined. Optical quality ofthe bottles was also visually noted. The results of these tests aretabulated in Table IV below.

                  TABLE IV                                                        ______________________________________                                        SUMMARY OF EXAMPLES 10 TO 14                                                  Wt. %      RAN (ppm)                                                          Ex.   Myrcene  Preform  Bottle Bottle Optical Quality*                        ______________________________________                                        10    0.0      9.1      8.3    A                                              11    1        --       1.9    A                                              12    1        2.9      3.9    A                                              13    1        3.7      3.6    A                                              14    1        5.3      3.4    A                                              ______________________________________                                         *A = control                                                             

The above data illustrates the efficiency of myrcene in reducing theamount of RAN in the walls of preforms and containers formed of styrenicresins containing polymerized acrylonitrile as a comonomer, in somecases to less than 3 ppm in such preforms and bottles. As indicated, theadditional minimal heat history experienced by the preforms in reheatingto bottle blowing temperatures did not significantly affect RAN level,and in all examples bottle optical quality was equal to that of thecontrol.

The preceding description is set forth for purposes of illustration onlyand is not to be taken in a limited sense. Various modifications andalterations will be readily suggested to persons skilled in the art. Itis intended, therefore, that the foregoing be considered as exemplaryonly and that the scope of the invention be ascertained from thefollowing claims.

What is claimed is:
 1. In the process of melting a styrenic polymercontaining at least about 10 percent by weight of polymerized styrene byphysical working while generating free styrene monomer in the melt, theimprovement which comprises having a scavenging amount of myrcene inadmixture with the polymer during said melting to minimize the level offree styrene monomer in the polymer.
 2. The process of claim 1 whereinthe physical working is accomplished by masticating the polymer in aplasticizing zone.
 3. The process of claim 1 wherein the styrenicpolymer and myrcene are dry blended together before said physicalworking.
 4. The process of claim 1 wherein the amount of myrcene is inthe range of from about 0.005 to about 2% based on the weight of thepolymer.
 5. The process of claims 1, 2, 3 or 4 wherein the styrenicpolymer contains acrylonitrile as a copolymerized monomer and themyrcene scavenges free acrylonitrile monomer during formation of themelt.
 6. The process of claim 5 wherein the polymer is masticated in aplasticizing zone comprising a screw rotating within a barrel.
 7. Theprocess of claim 5 wherein the polymer comprises from about 10 to about50 weight % polymerized styrene and from about 90 to about 40 weight %copolymerized acrylonitrile.
 8. A method for reducing the level of freestyrene monomer in a shaped product formed of a styrenic polymercontaining at least about 10 percent by weight of polymerized styrenewhich comprises having a scavenging concentration of myrcene in intimateadmixture with the polymer during at least one forming operation inwhich the polymer is heated to form a melt and shaped into such product.9. The method of claim 8 wherein the forming operation includes anextrusion operation.
 10. The method of claim 8 wherein the polymercomprises at least 20 weight percent polymerized styrene.
 11. The methodof claim 10 wherein the polymer contains acrylonitrile as acopolymerized monomer.
 12. The method of claim 11 wherein the myrcenealso scavenges free acrylonitrile monomer during formation of the melt.